What causes head gaskets to blow?

Is it the wrong viscosity engine oil? or something else

Several reasons......

Not changing coolant at regular intervals....corosion.
Overheating.....coolant leaks..faulty t'stat....clogged radiator ( internal or external ).
Reusing old head bolts.....TTY bolt ( Torque To Yeild )..use once only.
Surfaces...Block or Head...not flat..pitting..warped...twisted....cracked....coroded.
Surfaces...too smooth/shiny...need to be a certain roughness/surface finish.
Surfaces...... not clean enough when gasket fitted.
The procedure of fitting the gasket, cyl head, which adhesives to use where, tensioning bolts, bleeding cooling system..etc

Revving the s**t out of it when you just started the engine while the engine is still cold...

phong =P~

And the number one E-series reason is:
Wrong head gasket materials used.

Alloy expands more than steel, so having a long block and head doesn't help.

The MLS gaskets have aided in this and reduced the number Majorly.

Why say that alloy heads/cast iron blocks.....problem....what a load of crap!!!!!!
Alloy heads have been around since Jesus played fullback for Jersalem.
You hardly ever heard about it with the XD to XF alloy heads. My ol man had an XD for 13 years, never had a problem with it. My EA had the factory gasket on it for about 15 yrs, and I changed the head to a performance unit.
Like I said before...hardly anybody changes their coolant. A lot of probs started with the EF's...changes in maintence schedules, they dropped coolant & brake fluid changes ( added in the list ), and stretched out the major service from 40K to 50K intervals.
There 2 types of coolant in the EF's....R1-3B ( Blue ) and the R1-33 (??? i think ) green.
The blue stuff was just an inhibitor, the green was antifreeze/antiboil/inhibitor.
The blue stuff didn't last very long. The GLi's were filled with blue stuff and the models up from that had the green stuff.

my 2 cents

There is a lot of arguments about gaskets yes, and sure,
I don't doubt everything you say, but composite gaskets are nearly gone,

Try no to get stuck in the eighties, nearly all manufacturers use MLS now, it all comes down to contact.

There is nearly no squish, no delamination, no impregnated foreign substances in MLS and the list goes on.

As above, sure, lack of maintenance, low quality coolants kill engines.

There are Many web side explaining why and how.

http://www.eurotuner.com/techarticles/e ... index.html
First one i find on google.

I had a Similar reaction when I was informed by a reputable engine builder why NOT to use a 'off the shelf' mono torque gasket, and after a fair amount of research , Including a leading Melbourne ford dealers workshop, My argument came to a halt..

In my personal experiences, head gasket failure has been intimately linked to faults within the cooling system. Cracked radiators (specially the plastic bits), bursted hoses, corroded welsh plugs etc. One of my mates even did one by forgetting to put the radiator cap on. If its not leaking visibly, replace the thermostat and maybe temperature sender. Check your clutch fan is still sucking air through the radiator, and keep a very close eye on temperature. I reckon if all that does not fix the problem, it could be a stuffed radiator...
Good luck with it hey.

Why did car manufactures go away from metal head gaskets?????????.....and they had high compression ratio's back then too....then they went to composite gaskets....now to metal gaskets again.